The present invention relates to a method for the reduction of receive data of a radar, in particular, a synthetic aperture radar (SAR) and to a receiver for data of a radar, in particular, and SAR according to the independent claims.
On flying radar platforms, the reduction of received radar data is a necessity resulting from the mission requirements on the one hand and the limited downlink capacities on the other. The data reduction may also be advantageous for radar systems in other applications, for example, to reduce the cost of archiving.
An example for a flying radar platform is the SAR which, for example, samples the surface of the earth by way of a moving antenna. The SAR transmits transmit pulses for the sampling and receives the radar echo pulses. Thus, the transmit pulses reflect on the sampled objects (targets). For each area illuminated and sampled by the antenna, a SAR processor calculates, by respectively processing the data of the echo signals (target echoes), an image of the sampled area. A SAR is, for example, used to measure and image the earth surface. The transmit pulses transmitted by a SAR are typically chirp pulses, in particular, linear frequency modulated pulses. A satellite, or a different aircraft or a different flying object flying at a sufficient height above the earth surface or the surface of a planet, may be used as a flying platform for a SAR.
FIG. 1 illustrates the aspects relevant for a data reduction of the (radar) echo signals or receive data for a flying SAR platform. Area 16 (rectangular area in FIG. 1 which lies within the oval area) desired for the image generation is defined within area 14 (oval area in FIG. 1) which is illuminated by transmit pulses 10 of a SAR system 12. As a result, there is a closest and a furthermost target 18 or 20, the latter of which may be present multiple times depending on the geometry of the scene. An echo signal 22 or 24 from the closest and the furthermost target is obtained in the receiver of SAR system 12 such as it was generated in the transmitter of SAR system 12. A possible Doppler shift may here be ignored. All echoes emanating from targets which lie in between are situated in between said targets. Targets which lie outside of the desired area, yet are still inside the illuminated area, display chirps in the receive signal which lie in front of the closest or after the furthermost target.
FIG. 2 illustrates the assignment of the targets to the receive signal in a time-frequency diagram of the frequency spectrum of the received radar echo signals. The bandwidth is determined by the spectrum of the chirp signals used as radar signals. The receive window for the radar echo signals is a function of the desired area 16. In other words, radar echo signals are received from desired area 16 within the receive window. In order to evaluate the radar echo signals, only the dark-shaded signal part is needed. The two light strips which laterally delineate the dark-colored signal part include target echoes of targets outside desired area 16; signal energy from these areas does not enter the SAR product. The light, hatched (triangular) areas in FIG. 2 include merely reception noise and no signal energy from target echoes.
Thus far, digital methods for the reduction of data only have been used to a very limited extent in space missions. The reason is that the radiation resistant, electronic components for the required data rate required for the digital data reduction so far have not been available.
An essentially natural method for data reduction is the adaptation of the sampling rate of the radar echo and the corresponding anti-aliasing filter to the used bandwidth. This method is, for example, applied to the satellites TerraSAR-X, TanDEM-X and PAZ. In this instance, three different sampling rates are available:                110 MHz for 100 MHz of signal bandwidth;        165 MHz for 150 MHz of signal bandwidth; and        330 MHz for 300 MHz of signal bandwidth.        
Analogue anti-aliasing filters put in place upstream ensure the spectral limitation of the receiver noise.
Another data reduction method which has been used for a long time is deramping corresponding with the transformation of a time-frequency plane which enables extracting the desired signal by way of low-pass filtration. Unfortunately, deramping is only useful in very special cases, as illustrated in FIG. 3. In the time-frequency diagrams or spectrograms, the white area represents the reference data of a SAR echo in the time-frequency domain, while the grey rectangle represents the data to be recorded. If the ratio of the chirp-pulse length to the swath width is large (diagram above left), the deramping may significantly reduce the data volume (diagram lower left). In the case of a wide swath (right), there is no benefit.
Both methods described above use the characteristics of the receive signal only to a limited extent. An instrument having a plurality of different SAR modes requires a significantly more flexible method to reduce data.
An object of the present invention is to enable the improved reduction of data of a radar, for example, a SAR.
An underlying idea of the present invention is to specify a temporal receive window as a function of an area which is to be detected by a radar, in particular, as a function of a swath of a SAR, for a received radar echo signal of an emitted radar signal, which is a chirp-like transmit signal; to divide the received radar echo signal into spectral sub-bands; and to determine, for each spectral sub-band, a sub-band window which lies within the previously-specified temporal receive window. In doing so, the overall information content of interest of the received radar echo signal is included in the sub-band windows. The sub-band windows are activated within the temporal receive windows as a function of the receive time of the radar echo signal. The sampling rate for sampling the received radar echo signal is then adjusted as a function of the number of the sub-band windows active at the respective sampling instance. In other words, the procedure according to the present invention described above implements a variable sampling rate, as a result of which the data of the radar may be reduced. For this purpose, a chirp-like transmit signal is to be understood as a transmit signal which masks one or a plurality of frequency areas; that is, which has a time dependent transmit frequency. In this instance, linear and nonlinear, in particular, exponential chirps or chirp signals are typical examples. More generally, transmit signals, the spectrogram of which features a deterministic characteristic across one or a plurality of frequency ranges, are suitable for the purposes of the present invention. For the present invention, for example, a signal the spectrogram of which does not have a linear but a cloud-type curve would also be suitable. In this instance, it is only substantial that the short-term spectrum of the transmit signal shifts deterministically during the course of the transmit pulse so that it is possible to specify the sub-band window according to the receive signal emanating from the chirp-like transmit signal in such an appropriate manner that the overall information content of interest of the received radar echo signal is included in the sub-band windows and that, in this instance, at least one part of the sub-band window is shorter than the overall radar echo signal.
One embodiment of the present invention relates to methods for the reduction of receive data of a radar having the following steps: receiving a radar echo signal emanating from a chirp-like transmit signal; specifying a temporal receive window of the radar echo signal as a function of an area to be detected by the radar; dividing the radar echo signal into a plurality of spectral sub-bands; determining of sub-band windows for each spectral sub-band; activating the sub-band window within the temporal receive window of the radar echo signal as a function of the receive time of the radar echo signal; and sampling the radar echo signal by a sampling rate which is adjusted as a function of the number of the sub-band windows active at the respective sampling instance.
In particular, switch-on and switch-off instants for activating the respective sub-band windows are selected in such a manner that each sub-band window masks a temporal receive area of the radar echo signal in which the radar echo signal in the respective sub-band window includes information about the area to be detected by the radar.
The spectral sub-bands may be selected in such a manner that they have spectral ranges approximately equal in size.
Furthermore, a baseline sampling rate FS/M may be specified as a function of the spectrum of the radar echo signal and the number M of the spectral sub-bands and the sampling rate may be adjusted as a function of the baseline sampling rate.
The division of the received radar echo signals into a plurality of spectral sub-bands may be carried out in such a manner that each spectral sub-band directly borders at least one adjacent spectral sub-band.
In particular, the sampling values of the radar echo signal are, in conjunction with the switch-on and switch-off instants for activating the sub-band window, output as reduced receive data.
The reduced receive data may be further reduced by a block adaptive quantization.
A further embodiment of the present invention is a method for processing receive data of a radar, reduced by a method according to the present invention and as herein described, in which the signal represented by the receive data is digitally filtered by a filter bank configured to divide the signal into spectral sub-bands and to reduce the sampling rate according to the number of spectral sub-bands.
A modified Discrete Fourier Transform (DFT) filter bank is particularly used as a filter bank.
A further embodiment of the present invention relates to a computer program having a program code for carrying out all method steps according to the present invention and as herein described, if the computer program is carried out in a computer.
Furthermore, one embodiment of the present invention relates to a data carrier on which, according to the present invention and as previously described, the program code of the computer program executable by a computer is stored.
A further embodiment of the present invention relates to a device to reduce receive data of a radar having a receiver for a radar echo signal emanating from a chirp-like transmit signal, the receiver being configured in such a manner that a temporal receive window of the radar echo signal is specified as a function of an area to be detected by the radar, and a data reduction means for dividing the radar echo signal into a plurality of spectral sub-bands, for determining sub-band windows for each spectral sub-band, for activating the sub-band window within the temporal receive window of the radar echo signal and for sampling the radar echo signal by a sampling rate which is adjusted as a function of the number of the sub-band windows active at the respective sampling instance.
The means to reduce data may be particularly configured to carry out a method according to the present invention and as herein described.
A further embodiment of the present invention relates to a device for processing receive data of a radar which have been reduced by a device for reducing receive data of a radar according to the present invention and as herein described, having a filter bank for digitally filtering the signal represented by the receive data, and the filter bank is configured to divide the signal into spectral sub-bands and to reduce the sampling rate according to the number of spectral sub-bands.
The filter bank is particularly a modified DFT filter bank.
Further advantages and application possibilities of the present invention result from the subsequent description in conjunction with the exemplary embodiments illustrated in the drawings.
The description, the claims, the abstract and the drawings use the terms and assigned reference characters of the appended list of reference characters.